1. Field of the Invention
The embodiments herein generally relate to thermometry, and, more particularly, to methods for controlling growth temperatures in a molecular beam epitaxy system.
2. Description of the Related Art
Molecular Beam Epitaxy (MBE) is one of a family of methods used to grow single-crystal films on single crystal substrates (epitaxy). In an MBE system, a substrate, on which the crystalline film is to be grown, and several material sources are contained in an ultra-high vacuum chamber. The sources are typically furnaces, each containing a specific element to be deposited on the substrate. A furnace uses an open crucible in which the evaporant is placed. Moreover, resistive heating wires and heat shields generally surround the crucible. Generally, the evaporant is heated to a temperature that produces a material flux of desirable magnitude from the crucible opening (a molecular beam) and which is aimed towards the substrate crystal, where the molecules are allowed to condense. The beam is turned on and off by a mechanical shutter blade in front of the crucible opening. High stability of the flux, and therefore growth rate, is accomplished by high stability of the furnace temperatures.
In addition to stable source temperatures, a stable process relies on a well-controlled substrate temperature. The substrate is warmed by a resistively heated element placed behind it. Because the substrate should be allowed to rotate around its azimuthal axis during deposition to ensure good layer uniformity, there are generally inadequate solutions to mechanically contact the substrate for measurement of the temperature. Usually, a thermocouple is placed somewhere behind the substrate in the vicinity of the back side and the heater element. A substantial difference between the real temperature of the substrate front side and the thermocouple reading is therefore commonly observed. To achieve a stable temperature, a proportional integrating derivative (PID) control unit is typically used, which sets the output level from a power supply to the substrate heater, depending on the thermocouple reading. In many systems, the temperature setpoints are set by a digital control computer, which also opens and closes the mechanical shutters in front of the evaporation sources for predetermined times, thus producing specific film thicknesses.
The difference between the thermocouple reading and the true temperature is generally determined empirically. The most common way of obtaining more reliable temperature readings is to use an optical pyrometer. Typically, it is first calibrated by observing some type of phase transformation that is known to take place at a well-defined temperature. The pyrometer is then adjusted by setting a value for the apparent emissivity of the substrate so that the instrument reads the desired value.
It is possible to use the pyrometer signal directly as input in the feed back loop. This could be accomplished by hard wiring the pyrometer to the PID controller, or by feeding it to the control computer and letting it send appropriate signals to the PID controller. However, in either case, the use of the pyrometer is limited to temperature ranges above several hundred degrees, typically above 400° C. Since the substrate is at room temperature at the start of the process, pyrometers are generally not useful during the initial warm up phase. Also, during the deposition phase some materials may require deposition at temperatures below 400° C. In addition, some films and some holders of small substrates are prone to let stray light from the evaporations sources enter the pyrometer, which can produce erroneous readings. In these cases thermocouple control, or constant power output are the preferred choices.
Finally, deposition of layers that aim to produce structures with optical interference properties, such as a Bragg mirror, can be used. During deposition of such films the signal reaching a pyrometer as well as the temperature observed by the thermocouple exhibit strong oscillations, making both generally unreliable and unsuitable for feedback control. Accordingly, there remains a need to improve the MBE deposition sequence via more accurate temperature control.